The subject invention relates to spectroscopy ellipsometry. More specifically, the subject invention relates to an ellipsometer which utilizes an all-refractive optics system for focusing a broadband probe beam onto a sample.
Ellipsometry is a powerful technique for evaluating thin films on semiconductors. In any ellipsometer, a probe beam having a known polarization state is directed to interact with a sample. Changes in the polarization state induced by the interaction of the probe beam with the sample are then monitored. This information, in the form of xcexa8 and xcex94 measurements, is used to analyze characteristics of the sample.
It is well known that in order to improve an ellipsometric analysis, some form of multiple measurements are desirable. This can include taking measurements at multiple angles of incidence or at multiple wavelengths. Spectroscopic ellipsometers using monochrometers to scan the wavelength have been known for many years. For example, see xe2x80x9cHigh Precision Scanning Ellipsometer,xe2x80x9d by Aspnes and Studna, Applied Optics, Vol. 14, No. 1, January 1975.
More recently, efforts have been made to expand the desired wavelength range of spectroscopic ellipsometers into the UV and to obtain measurement data at multiple wavelengths simultaneously. When used in the semiconductor field, broadband probe beams must be focused to a relatively small spot size on the sample surface. Attempts to use lenses (refractive optics) to focus broadband probe beams, and particularly those including UV wavelengths, have run into problems. First, lenses typically have chromatic aberrations. Such aberrations cause the focused distance to be different for different wavelengths. Another problem with lenses is that it was often difficult to find lens materials with good transmission characteristics across a broad wavelength range.
Due to these difficulties, researchers in the prior art began using curved mirrors to focus the broadband probe beam onto the sample surface. Mirrors are advantageous since they can be highly reflective across a broad range of wavelengths. In addition, mirrors exhibit little or no chromatic aberrations. The use of focusing mirrors for a broadband ellipsometer are described in U.S. Pat. Nos. 4,790,659, issued December 1988, to Erhman, and 5,608,526, issued Mar. 4, 1997, to Piwonka-Corle.
Unfortunately, while providing a solution for chromatic aberration, mirrors are relatively difficult to align. More specifically, since mirrors must be focused off-axis, any angular error in alignment creates twice that error in beam position. Further, if the mirror is removed from the optics path, a light beam cannot be used to align the rest of the optics path since the mirror is needed to turn the beam. In contrast, the elements of an optics system can be aligned when a focusing lens is removed from the beam path. Another problem with mirrors is that when the probe beam reflects off the mirror, the polarization state of the beam is changed. Such changes must be very accurately and precisely characterized, otherwise they will cause errors in the analysis of the sample.
Accordingly, it would be desirable to have an all-refractive (lens-based) system for focusing a broadband probe beam to a small spot onto a sample surface. Such a system would be easier to align. In addition, any polarization changes in the probe beam induced by the lens system can be more accurately controlled.
In accordance with these objects, the subject invention provides for a spectroscopic ellipsometer which can obtain a small focused beam spot on a sample using all-refractive optics. The spectroscopic ellipsometer is of the type which has at least one broadband light source emitting both UV and visible wavelengths. The source typically has a range of at least 500 nm and preferably covers a wavelength range of about 200 nm to 800 nm.
Light from the broadband source is polarized and then focused onto the sample with an all-refractive optical system. The optical system includes at least two lenses which are transmissive in the UV and visible wavelengths. The curvatures of the lenses are selected to minimize chromatic and spherical aberrations. With respect to the chromatic aberration compensation, the variation in the focal point over the wavelength range (focal shift) should be no more than five percent and preferably less than 2.5 percent of the mean focal length of the optical system. The optical system is intended to focus the beam on the sample to a small spot, on the order of 3 mm or less.
An analyzer system is provided for monitoring the change in polarization state of the reflected beam induced by interaction with the sample. Any conventional analyzer can be used, including rotating analyzer or rotating compensator (waveplate) configurations. In the preferred embodiment, an aperture or spatial filter is provided in the path of the reflected beam to limit the area of the sample investigated. In the preferred embodiment, the spatial filter includes a focusing optical element and an aperture configured to limit the measured region to a spot size of 100 microns or less.
By using an all-refractive optical system for focusing the beam on the sample, problems associated with the alignment of mirrors are eliminated. In addition, the problems associated with changes in the polarization state of the beam induced by the mirrors are also eliminated.
Further objects and advantages will be apparent from the following detailed description taken in conjunction with the drawings in which: